lec 4- The structure and function of the plasma membrane Flashcards
what is the plasma membrane?
The outer boundary of the cell that separates it from the world, is a thin fragile structure
what are the 7 things a plasma membrane does?
-compartmentalization (forms continuous sheets that enclose intracellular compartments)
-scaffold for biochemical activities (provide a framework that organizes enzymes for effective interaction)
-selectively permeable barrier
-transporting solutes
-responding to external signals (receptors transduce signals)
-intracellular interactions
-energy transduction (transduce photosynthetic energy)
what is the lipid to cell surface area ratio that the lipid bilayer accounted for?
2:1
why is the surface tension of membranes lower than that of pure lipids?
due to protein being in the membrane
what stabilizes bilayers?
van der waals interactions between acyl chains, and ionic/ hydrogen bonds with polar heads
what varies depending on the type of membrane and the type of cell it is?
The ratio of lipid to protein
what is the ratio of lipid to protein in the inner mitochondrial membrane?
very high ratio
what is the ratio of lipid to protein in the myelin sheath?
low ratio
what diseases have defective lipids in membranes?
alzheimers and infant respiratory distress syndrome
what are the three major lipids in the membrane?
phosphoglycerides (most common/ AKA phospholipids)
sphingolipids (glycerol backbone)
cholesterol
what are the molecules that the phosphate in the head group of the phospholipid links to?
choline, ethanolamine, serine, and inositol
how many carbons do phosphoglycerides have?
16-22 carbons
what are the steps to lipid biosynthesis in the ER?
step 1: acyl transferases successively add two fatty acids to glycerol-3-phosphate to produce phosphatidic acid
step 2: a phosphotase removes the phosphate leaving Diacylglycerol (DAG)
step 3: the addition of a head group
what are the types of sphingolipids?
sphingosine = an amino alcohol that contains a long hydrocarbon chain
ceramide =sphingocine linked to a fatty acid
sphingomyelin = phosphorylcholine added to ceramide (also a phospholipid)
glycolipids = a carbohydrate added to a ceramide instead of a phosphorylcholine
what are the two types of glycolipids?
cerebrocide = one sugar added to ceramide
gangliocide = a cluster of sugars added
where is sphingolipid synthesis finished after it leaves the ER?
the golgi
where are glycolipids found in the membrane?
in the exoplasmic face of the membrane
what makes up 50% of the lipids inside the membrane?
cholesterol
what are liposomes?
vesicles
what does the asymmetry of membrane lipids do?
causes different physical and chemical properties in two different leaflets
in which direction do carbohydrates face in the plasma membrane?
face outward
what are integral proteins?
proteins that are penetrated and passed through the membrane (aka transmembrane proteins)
what are peripheral proteins?
proteins that stick on the outer side of the membrane
what are lipid-anchored proteins?
proteins on surface of cell attached to lipids, distinguished both by the types of lipid anchor and their orientation
what do integral membrane proteins do?
function as receptors, channels, proteins that transfer electrons
what is acylation of a lipid-anchored protein?
when the lipids in the palmitoyl group or myristol group attach to cytosolic proteins
what is prenylation of a lipid-anchored protein?
instead of fatty acid, protein is attached to prenyl group
what are examples of lipid anchored proteins?
G-proteins, acetylcholinesterase, Ras
what are transmembrane domains?
a string of about 20 mostly nonpolar amino acids that span the lipid bilayer as an alpha helix
what is glycophorin A?
a major integral protein of erythrocyte plasma membrane, 17 of 20 amino acids are hydrophobic that face the lipid layer
what anchors glycophorin A to the membrane?
positively charged arginine and lysine residues near the cytosolic face
what protein is one of the two that are exposed on red blood cells?
glycophorin A
which protein had its complete amino acid sequence determined first?
glycophorin A
what does a hydropathy plot do?
measure the hydrophobicity of amino acids
what is bacteriorhodospin?
a seven membrane-spanning alpha helical protein that acts as a transmembrane proton pathway
what protein generates gap junctions?
connexin
what are lipid rafts and what do they do?
the outer leaflet of plasma membrane has cholesterol and sphingolipids that pack together and form lipid rafts that float in the more disordered environment and they provide a favorable environment for cell surface receptors
are lipid bilayer molecules rapidly fluctuating?
yes
how long does it take for a phospholipid to move laterally down the same leaflet?
a few seconds
how long does it take for a phospholipid to move down to the other leaflet?
hours to days
what enzyme moves phospholipids in leaflets?
flippase
T or F:
membranes in fused cells cannot fuse
no
how can protein be labelled and tracked?
by FRAP (fluorescence recovery after photobleaching)
what are the steps of FRAP?
unlabelled cell surface gets labelled with fluorescent dye, laser beam bleaches an area on cell surface, dyed molecules diffuse into area, bleached area disappears
